1. Field of the Invention
This invention relates to a data communication system and more particularly to a system having an integrated circuit digital encoder which generates a standard telephone frequency DTMF output in response to input data.
2. Description of the Prior Art
The American utility telephone companies have developed a system of audio frequency data communication which is commonly called Touch Tone. The system provides an output signal which indicates one of 16 information states with a dual tone multiple frequency signal that combines one of four lower frequency signals with one of four higher frequency signals. Upon detection by a receiver this two of eight frequencies signal can be utilized to reconstruct the communicated data. Such systems are well known and are described inter alia in U.S. Pat. No. 3,076,059 to Meacham et al. and U.S. Pat. No. 3,128,349 to Boesch et al.
These DTMF systems typically utilize a keyboard with up to 16 keys which are decoded as one of four rows and one of four columns upon key actuation. Each row and column signal enables one of four LC tuned circuits to excite the desired two of eight output frequencies. Such systems provide adequate data communications but require expensive and complex mechanical and electrical components, are prone to failure and are much bulkier than systems according to the invention described herein. Furthermore, the prior art systems provide little in the way of flexibility or operator convenience, although a mechanical key rollover interlock may be implemented.